1. Field of the Invention
The present invention generally relates to an image forming apparatus, and specifically relates to a sheet alignment mechanism that aligns sheets after image formation.
2. Description of the Related Art
Conventionally, sheet post-processing apparatuses so called finishers have been known. One such sheet post-processing apparatus has been disclosed in Japanese Patent No. 2960770. The disclosed sheet post-processing apparatus includes a sheet alignment mechanism having a transport path along which a paper is transported, a stacking tray arranged at a predetermined angle in which sheets transported along the transport path are sequentially stacked, a pair of side fences that are symmetrically moved by a single drive source so as to align the sheets stacked on the stacking tray, and a stapler that staples a sheet bundle aligned in the stacking tray and detects, using a sensor, a home position, i.e., starting point, of one of the side fences.
Because the home position of the side fences is detected with one sensor, the structure is cost effective. However, if malfunction occurs in a drive system that moves the other one of the side fences not detected by the sensor or when the side fences are assembled with deviation, in many cases, sheets may not be aligned or may be stuck for some unknown reasons because no detecting unit is provided for such malfunction.
Furthermore, to move the side fences symmetrically with a single drive source, it may be configured such that the side fences are disposed symmetrically with respect to a drive pinion provided at the center and each of the side fences has a rack attached thereon to catch the pinion. Alternatively, the side fences may be fixed symmetrically on a timing belt placed in a sheet width direction. However, both of those structures suffer in that a gap between the side fences varies due to fluctuations in dimension error of components, the shift of their installation positions, or other factors, with the result that sheets are not aligned or are stuck for some unknown reason in many cases.